This invention relates to a low bit-rate pattern coding method and a device therefor. The low bit-rate pattern coding method or technique is for coding an original pattern signal into an output code sequence at low information transmission rates. The pattern signal may either be a speech or voice signal or a picture signal. The output code sequence is either for transmission through a transmission channel or for storage in a storing medium.
This invention relates also to a method of decoding the output code sequence into a reproduced pattern signal, namely, into a reproduction of the original pattern signal, and to a decoder for use in carrying out the decoding method. The output code sequence is supplied to the decoder as an input code sequence and is decoded into the decoded pattern signal by synthesis. The pattern coding is useful in, among others, speech synthesis. The following description is concerned with speech coding.
Speech coding based on a multi-pulse excitation method is proposed as a low bit-rate speech coding method in an article which is contributed by Bishnu S. Atal et al of Bell Laboratories to Proc. IASSP, 1982, pages 614-617, under the title of "A New Model of LPC Excitation for Producing Natural-sounding Speech at Low Bit Rates." According to the Atal et al article, speech synthesis is carried out by exciting a linear predictive coding (LPC) synthesizer by a sequence or train of excitation or exciting pulses. Instants or locations of the excitation pulses and amplitudes thereof are determined by the so-called analysis-by-synthesis (A-b-S) method. It is believed that the model of Atal et al is prosperous as a model of coding at a bit rate between about 8 and 16 kbit/sec a discrete speech signal sequence which is derived from an original speech signal. The model, however, requires a great amount of calculation in determining the pulse instants and the pulse amplitudes.
In the meanwhile, a "voice coding system" is disclosed in United States Patent Application Ser. No. 565,804 filed Dec. 27, 1983, by Kazunori Ozawa et al for assignment to the present assignee based on three Japanese patent applications which were laid open to the public under Japanese Paent Prepublications (Publications of Unexamined Patent Applications) Nos. 116,793, 116,793, and 116,795 in 1984. The voice or speech coding system of the Ozawa et al patent application is for coding a discrete speech signal sequence of the type described into an output code sequence, which is for use in a decoder in exciting either a synthesizing filter or its equivalent of the type of the linear predictive coding synthesizer in producing a reproduction of the original speech signal as a reproduced speech signal. The discrete speech signal sequence is divisible into segments, such as frames of the discrete speech signal sequence.
In the manner which is described in the above-cited Japanese patent prepublications and will later be described more in detail, the speech coding system of the Ozawa et al patent application comprises a parameter calculator responsive to each segment of the discrete speech signal sequence for calculating a parameter sequence representative of a spectral envelope of the segment. Responsive to the parameter sequence, an impulse response calculator calculates an impulse response sequence which the synthesizing filter has for the segment. In other words, the impulse response calculator calculates an impulse response sequence related to the parameter sequence. An autocorrelator or covariance calculator calculates an autocorrelation or covariance function of the impulse response sequence. Responsive to the segment and the impulse response sequence, a cross-correlator calculates a cross-correlation function between the segment and the impulse response sequence. Responsive to the autocorrelation and the cross-correlation functions, an excitation pulse sequence producing circuit produces a sequence of excitation pulses by successively determining instants and amplitudes of the excitation pulses. A first coder codes the parameter sequence into a parameter code sequence. A second coder codes the excitation pulse sequence into an excitation pulse code sequence. A multiplexer multiplexes or combines the parameter code sequence and the excitation pulse code sequence into the output code sequence.
With the system according to the Ozawa et al patent application, instants of the respective excitation pulses and amplitudes thereof are determined or calculated with a drastically reduced amount of calculation. It is to be noted in this connection that the pulse instants and the pulse amplitudes are calculated assuming that the pulse amplitudes are dependent solely on the respective pulse instants. The assumption is, however, not applicable in general to actual original speech signals, from each of which the discrete speech signal sequence is derived.
An improved low bit-rate speech coding method and a device therefor are revealed in United States Patent Application Ser. No. 626,949 filed July 2, 1984, as an elder or prior patent application by the instant applicant for assignment to the present assignee, based on two Japanese patent applications which were laid open to the public under Japanese Patent Prepublications Nos. 17,500 and 42,800 in 1985. It is possible with the method and the device according to the elder patent application or the last-mentioned Japanese patent prepublications to code an original speech signal into an output code sequence with a small amount of calculation and yet the output code sequence made to faithully represent the original speech signal.
According to the elder patent application, the sequence of excitation pulses is produced by using the autocorrelation and the cross-correlation functions in recursively determining instants and amplitudes of the excitation pulses with the instant of a currently processed pulse of the excitation pulses determined by the use of the instants and the amplitudes of previously processed pulses of the excitation pulses and with renewal of the amplitudes of the previously processed pulses carried out concurrently with decision of the amplitude of the currently processed pulse by the use of the instants of the previously and the currently processed pulses. Alternatively, the sequence of excitation pulses is produced by using the autocorrelation and the cross-correlation functions in recursively determining instants and amplitudes of the excitation pulses with the instant of a currently processed pulse of the excitation pulses and the amplitudes of previously processed pulses of the excitation pulses and of the currently processed pulsed determined by the use of the instants of the previously processed pulses.
Before coding the pulse amplitudes, it is desirable to quantize each pulse amplitude into a quantized pulse amplitude. This gives rise to a quantization error. In other words, the method and the device of the elder patent application have a quantization characteristic which has a room for improvement.